Nitridation mechanism of mesoporous silica: SBA-15

Mesoporous silicon oxynitride materials (NSBA-15) are prepared by the thermal treatment of mesoporous silica (SBA-15) in NH3 flow at various temperatures (between 873 and 1273 K), and are characterized by FT-IR, elemental analysis, ESR, XRD, 29Si MAS NMR, and N2 adsorption/desorption. The nitridation routes were determined through the investigation on surface defect species and N distribution around the tetrahedral site of Si as well as the surface groups of NSBA-15. The structural changes by nitridation are also investigated through the study on the changes in porosity under thermal treatments in NH3 or N2 flow. Our investigations have revealed the role of surface defect species as well as surface groups and NH3 in the nitridation. The nitridation proceeds through the radical reaction of NH3 with Si-O and Si species below 973 K. N-related defects are produced through the open reaction of siloxane bridging bonds by H atoms generated from NH3 on the surface above 1073 K. Nitridation is enhanced by the condensation of SiNH2 species and the radical reaction of NH3 with N-related defects. Based on the understanding of the factors that are effective in the nitridation, the nitridation mechanism of mesoporous silica is proposed.